'two-year' originates from the combination of the numeral 'two' and the noun 'year', where 'two' signifies the number 2 and 'year' refers to a period of 12 months.

'two-year' evolved from the Old English 'twegen gear', which meant 'two years', and eventually became the modern English term 'two-year'.

Initially, it meant 'a period of two years', and this meaning has largely remained the same in modern usage.